The present invention relates generally to gas generant compositions, especially gas generant compositions employed in various autoignition devices, such as vehicle occupant passive restraint systems (air bags), fire suppressants, aircraft escape chutes, life rafts and the like.
Auto-ignition and ignition materials are used in many gas generator devices such as protective passive restraints or air bags used in motor vehicles, escape slide chute, life rafts, fire suppressant canisters, and the like, the inflation devices of which are normally stored in a deflated state and are inflated with gas substantially instantaneously at the time of need. Such devices are often stored and used in close proximity to humans and, therefore, must be designed with a high safety factor that is effective under all conceivable operational conditions.
Inflation is sometimes accomplished solely by means of a gas generant composition and its"" associated ignition devices. At other times, inflation is accomplished by means of a gas or mixture of gases, such as air, nitrogen, carbon dioxide, helium, and the like, which is stored under pressure, and further pressurized and supplemented at the time of use by the addition of high temperature combustion products produced by the combustion of gas generative compositions and their associated auto-ignition and ignition compositions. The use of a stored, pressurized gas in conjunction with a supplemental gas generative composition is often referred to a xe2x80x9chybrid systemxe2x80x9d, since it is neither purely stored gas, nor solely reliant on a gas generative composition alone to accomplish inflation. Stored gas pressure in these hybrid inflators can sometimes reach 4,000 psi and greater. As will be discussed later, this condition is an important factor in the present invention. Note that the current invention will be especially useful in all hybrid inflators, whether the stored gas is inert (i.e, nitrogen, helium, argon, etc.) or whether the stored gas is oxygenated (i.e., contains some oxygen in addition to inert gases) to supplement fuel-rich exhaust products from the gas generator.
It is, of course, critical that the gas generative composition be capable of safe and reliable storage without decomposition or ignition at temperatures that are likely to be encountered in a motor vehicle or other storage environment. For example, temperatures as high as about 70 to 85xc2x0 C. may be reasonably experienced under extreme operational conditions in the field. Further, quality assurance testing during the manufacturing and testing process often requires even higher temperature exposures in the range of 107 to 115xc2x0 C. and greater. It is important that the gas generative device be thermally stable under these extreme environments where unexpected ignition could endanger people and facilities.
Ignition materials are commonly employed in these gas generative designs to safely ignite the gas generant when an electrical signal is received in response to an automobile impact or other stimulus. The ignition train, consisting of squib, initiator, booster material, auto-ignition device, and other secondary ignitors, must also be thermally stable at the extreme temperatures described above. In certain cases, the subject auto-ignition device may be part of the ignition squib device, separate from the other ignition components, part of the primary or secondary ignitor, or may make up the entire primary and/or secondary ignitor charge depending on the inflator design.
Generally, the air bag inflator, or other related devices, must exhibit benign response to environments wherein the decomposition temperature and gas generation of the primary gas generant, or a significant portion thereof, is reached. This condition would occur in the event that the device is exposed to a fire or high heat condition, such as might develop after an automobile crash or similar event.
Following slow or rapid heating to the decomposition temperature, most air bag inflation devices will decompose so rapidly that over-pressurization and explosion of the device is likely. To prevent this potentially life-threatening condition, inflation devices are often equipped with an auto-ignition material or propellant (hereafter referred to as xe2x80x9cAIPxe2x80x9d), designed to ignite at a temperature substantially lower than the decomposition temperature of the main gas generative composition. The AIP is usually present in small charges such that when the AIP ignites during a fire or other heating condition, a catastrophic explosion does not occur, but rather the AIP benignly burns and ignites one or more of the components in the ignition train or the main gas generant. The AIP is preferably located within the inflator in an area that is most conducive to thermal conductivity and/or to provide the desired performance characteristics.
As is noted below, where the gas generative composition is subject to melting prior to decomposition, it is desirable that the AIP device functions prior to reaching the melt temperature, as this avoids unpredictable and potentially catastrophic rapid burning and over-pressurization of the liquid components. As will be seen, this is a potential problem with certain gas generative compositions based on ammonium nitrate solid solution and eutectic mixtures.
A review of the art from the past decade shows an initial movement away from highly toxic azide-based gas generative compositions. New, low-cost, lower toxicity, more efficient clean burning replacements for the old azide-containing compositions were sought (see U.S. Pat. No. 6,017,404 to Lundstrom et al and U.S. Pat. No. 5,883,330 to Yoshida). Main gas generative formulations exhibiting higher melt temperatures offered an advantage when selecting an AIP formulation since theory suggests that the AIP must ignite prior to the melting point of the main gas generative composition in order to survive slow cook off. Thus, higher melting points would permit the formulator to select more easily tailored higher auto ignition temperature AIP mixtures. For the higher melting gas generative formulations developed under these goals, many AIP formulations have been tailored to meet higher temperatures in the range of 150 to 180xc2x0 C. and higher (see U.S. Pat. No. 5,084,118 to Poole).
The search for clean, low-cost oxidizers led to the development of ammonium nitrate (AN)-based formulations. However, some of these formulations suffered from inadequate thermal-cycling stability due to the well-known problems associated with a phase change and volumetric shifts. This problem sometimes led to dimensional instability and grain cracking, which caused the ballistic properties of gas generative device to degrade. In an effort to resolve this problem, the use of certain blended oxidizer systems, wherein AN solid solution and eutectic mixtures were employed, were developed (see U.S. Pat. No. 5,850,053 to Scheffee et al and U.S. Pat. No. 5,411,615 to Sumrail et al).
One drawback to the eutectic mixtures and solid solutions with AN was the aforementioned low melting point characteristic. These formulations often exhibited melting points in the range of 120 to 130xc2x0 C. This fact, along with the need for new, lighter weight pressure vessels made out of aluminum which suffered from severe strength losses at higher temperatures (see U.S. Pat. No. 5,084,118 to Poole), motivated the industry to search for new AIP mixtures that would provide ignition temperatures in the range of 130 to 170xc2x0 C.
Initial attempts at development of new low temperature AIP to meet this criteria made use of (1) effective catalyst combined with AP/fuel mixtures (see U.S. Pat. No. 5,763,821 to Wheatley), (2) chlorate-based mixtures in combination with organic sugars and organic acids (see U.S. Pat. No. 5,460,671 to Khandhadia), and (3) low melting oxidizers to increase reactivity of the mixture at the melt zone (see U.S. Pat. No. 5,886,842 to Wilson et al). Recently, U.S. Pat. No. 5,739,460 to Knowlton et al disclosed the use of molybdenum fuels in combination with low melting oxidizers based on silver nitrate to achieve lower auto-ignition temperatures.
Clean, fast-burning, self-deflagrating fuels have also been proposed that could be used as a main constituent of the gas generative composition, or, if the auto-ignition temperature were low enough, that could be used in a new family of AIP compositions (see U.S. Pat. No. 5,811,725 to Klager, U.S. Pat. No. 6,093,269 to Lundstrom et al, and U.S. Pat. No. 6,143,101 to Lundstrom). Compounds containing azo-functional groups were identified as potentially fast burning fuels. U.S. Pat. No. 6,093,269 to Lundstrom et al identified a new type of azo-functional compound for gas generant devices. This compound, azobisformamidine dinitrate, also known as azodiformamidine dinitrate, or azodicarbonamidine dinitrate (all three hereafter=AZODN) as described in the Lundstrom et al ""269 patent, proved to be a clean, fast-burning compound with a high oxygen content. It also exhibited an inherently low decomposition point in the range of 170 to 180xc2x0 C. depending on test method, yet was thermally stable over the severe temperature conditions of the automotive airbag specifications. As described in U.S. Pat. No. 6,143,101 to Lundstrom, this compound provided the basis for a new family of AIP compositions having an auto-ignition temperature in the range of 150 to 170xc2x0 C.
However, use of the low-cost, AN-based eutectics and solid-solutions for gas generative compositions in the hybrid devices wherein the stored gas pressures are nominally between 3,000 and 4,000 psi at ambient temperatures, created the need for a new, more aggressive AIP composition. In these hybrid systems, the pressure effect on the AIP resulted in decreased thermal stability such that AIP compositions that were formerly stable at ambient pressures, now failed to meet the thermal soak criteria under pressure. In many AIP compositions, the gap between the auto-ignition temperature and the maximum temperature to meet thermal soak widened. Although not thoroughly understood, it is believed that the gas pressure may confine or imprison volatile, auto-catalytic-decomposition products that would otherwise escape, thus reducing the stability of the AIP to long exposure to high temperature. A similar effect has been noted where compositions are thermally stable when vented to the atmosphere, but are thermally unstable in the same environment when hermetically sealed. This effect has been especially pronounced in certain formulations containing ceric ammonium nitrate.
Due to these and other factors, none of the AIP compositions generally noted above were able to meet the severe conditions imposed by the hybrid environment, while still meeting the auto-ignition needs of the gas generative device to fire or other high temperature conditions. The AZODN-based mixtures for use with AN-based eutectics and solid solutions did not offer a low enough auto-ignition temperature. The molybdenum-based mixtures were not thermally stable under pressure at standard inflator test conditions (i.e., 107 to 115xc2x0 C.), and could not be safely compacted into a pellet form without suffering decomposition during long-term thermal storage conditions. The chlorate- and AP-based mixtures proved to be especially susceptible to the pressure effect, causing large shifts in thermal soak and auto-ignition temperatures. Chlorate-based mixtures were generally not desirable anyway due to concern for the formation of ammonium chlorate when used with AN-based systems, and their sensitivity to contamination by certain organic salts and acids.
Thus, the current art does not satisfy the needs of the new hybrid and dual hybrid (xe2x80x9csmartxe2x80x9d airbags) gas generator designs, where these designs incorporated the low-melting AN-based eutectics and solid solutions. These designs needed a new AIP that was thermally stable under pressure at temperatures up to the range of 115 to 130xc2x0 C., and yet would ignite rapidly at temperatures between 130 and 150xc2x0 C. None of the above approaches offered this sharp temperature transition between thermally stable and auto-ignition conditions when under pressurized conditions, where the auto-ignition temperature was in the range of 130 to 150xc2x0 C.
Broadly, the present invention is related to gas generant compositions which exhibit low autoignition temperatures. In preferred forms, the present invention is embodied in gas generant compositions which are comprised of azobisformamidine dinitrate (AZODN) and a low-melting oxidizer which includes a eutectic or solid solution of two or more nitrate or perchlorate salts. A low-melting oxidizer comprised of silver nitrate and potassium nitrate is preferred in the formulations of the invention in an amount to achieve a low autoignition temperature of between about 116xc2x0 C. (241xc2x0 F.) to about 150xc2x0 C. (302xc2x0 F.).
The compositions of the invention may include a variety of auxiliary components typically employed in conventional gas generant compositions for their intended purpose. For example, especially preferred formulations of the present invention will include a powdered metal or metal oxide as a combustion catalyst to speed the decomposition reaction and also as a combustion aid to facilitate the ignition of the primary propellant or gas generant.
The current invention is directed to meet these goals and provide a substantially azide-free and chlorate-free auto-ignition composition. Specifically, the invention is especially embodied in an azide- and chlorate-free composition that is comprised of (i) the low auto-ignition fuel, AZODN, (ii) a low melting oxidizer mixture comprised of binary, tertiary, or ternary eutectic or solid solution mixtures of nitrate and/or perchlorate salts, (iii) a low melting organic fuel that lowers the auto-ignition temperature and also provides acid scavenging (thermal stabilizer) effect as in n-MNA, and (iv) a catalytic metal oxide powder. In this invention, one function of the acid scavenger is to react with and render neutral various auto catalytic species which, if left in the composition, will promote more rapid decomposition and reduce the useful shelf-life of the AIP mixture. One especially preferred AIP composition in accordance with the current invention includes AZODN, the binary solid solution of silver nitrate and potassium nitrate, n-MNA, and super-fine iron oxide (NANOCAT). These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.